Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Basecoat-clearcoat composite coatings are topcoats that offer exceptional gloss, depth of color, distinctness of image, or special metallic effects. The automotive industry has made extensive use of basecoat-clearcoat composite coatings for automotive body panels. Single layer topcoats and the clearcoats of color plus clear composite coatings usually require an extremely high degree of clarity and gloss to achieve the desired visual effect. Furthermore, they must maintain the clarity and gloss over long periods of time in the face of environmental challenges.
Clearcoat coating compositions used as the outermost automotive coating are subject to damage caused by numerous elements. These elements include environmental fall out, exposure to ultraviolet radiation from sunlight, exposure to high relative humidity at high temperature, and defects made by impacts of small, hard objects resulting in chipping. “Environmental etch” is a term applied to a kind of exposure degradation that is characterized by spots or marks on or in the finish on the coating that often cannot be rubbed out.
In order to be commercially successful, a coating should provide as many favorable characteristics as possible. In addition to etch resistance and resistance to chipping, it is desirable to provide curable coating compositions with favorable viscosity properties so as to exhibit good rheology or sag control. Furthermore, coatings are required which exhibit good properties of intercoat adhesion and which are resistant to defects that occur during the curing process such as popping. Also, it is desirable to provide coatings that maintain their good coating properties at temperatures involved in high bake repair.
Resins for coating compositions are selected by the formulator to provide desirable properties associated with the structure of the resin. Because most resins require curing before development of optimum coating properties, it is usually necessary for the formulator to add a crosslinking agent or resin to the coating composition. The crosslinking resin, in general, contains reactive functional groups that react with the functional groups of the coating resin. In many cases, it is advantageous to provide crosslinking resins that are compatible with the coating resin and that have as high a functionality as possible. Such crosslinkers can be used in relatively small amounts without detracting from the coating properties of the composition.
Crosslinking agents with a low equivalent weight per carboxyl group have not been widely used in the coatings industry because it has been felt that the reactivity and catalytic effect of the carboxyl groups would lead to undesirable coating properties. For example, in U.S. Pat. No. 6,103,387 to Yamamoto et al., thermosetting compositions are provided that include a compound having two or more carboxyl groups blocked by a vinyl ether compound. The carboxyl groups of the compound are blocked so as to increase storage life and reduce reactivity. The patent teaches that when compounds containing carboxyl groups are used to produce topcoating compositions, high solids contents are difficult to achieve and the appearance of the finished coat is inferior.
In U.S. Pat. No. 4,477,624 to Waki, heat resistant synthetic resin compositions are provided that contain a polyamide imide type resin obtained by reacting a diisocyanate with a tricarboxylic acid containing citric acid. The reaction occurs at high temperatures for several hours, so that the carboxyl groups of the tricarboxylic acid react with the isocyanate groups of the diisocyanate.
It would be desirable to provide a crosslinking resin having carboxyl functionality and a low equivalent weight per carboxylic group. It would be further desirable to provide such a crosslinking resin by reaction of a hydroxy functional carboxylic acid with an isocyanate under mild reaction conditions. This invention provides such a solution.